1. Field of the Invention
The present invention relates to a method and apparatus for receiving DMB (Digital Multimedia Broadcasting) services, and more particularly to a method and apparatus for effectively receiving DMB services even in a weak electromagnetic field region.
2. Description of the Related Art
DMB services represent the next generation digital broadcasting services to enable the user to enjoy high quality multichannel multimedia broadcasting. These DMB services may be provided through a variety of wireless communications sets such as a mobile terminal, vehicle-mounted wireless terminal, and home based television set. DMB services are generally divided into satellite DMB and terrestrial DMB according to the method of transmitting information. Satellite DMB utilizes satellite communications to enable subscribers to freely enjoy various multimedia contents through wireless terminals with a directional antenna, such as a personal mobile terminal and a vehicle-mounted wireless terminal, while they are moving outdoors. Terrestrial DMB generally provides multimedia services through the allotted frequencies of 204 to 210 MHz, VHF band corresponding to TV Channel Number 12.
Referring to FIG. 1, which illustrates a conventional DMB system, satellite DMB services are provided by satellite DMB center 102 to send DMB signals of 13.824 to 13.883 GHz to satellite 100, which in turn sends TDM (Time Division Multiplex) signals of 12.214 to 12.239 GHz for the terrestrial radio station 104 and signals of 2.630 to 2.665 GHz directly for a personal wireless terminal. The terrestrial radio station 104 is a system for providing the radio signals to a wireless terminal existing in a region not capable of receiving the signals directly from the satellite. Namely, the terrestrial radio station 104, acting as gap filler, demodulates the TDM signals of 12.214 to 12.239 GHz received from the satellite 100, and then modulates them into CDM (Code Division Multiplex) signals of 2.630 to 2.665 GHz to be received by the mobile terminal in a multiple fading environment such as a city or metropolis.
The DMB receiving terminal 106 may be a personal mobile terminal, vehicle-mounted wireless terminal, or fixedly mounted communications set to receive the signals directly from the satellite 100 or through the terrestrial radio station or gap filler 104. The DMB signals received by the DMB receiving terminal 106 directly from the satellite 100 usually have a weak level. If the DMB receiving terminal 106 is far away from a metropolis with the gap filler 104, it can only receive the signals from the satellite. Accordingly the region where the signals can received only from the satellite 100 is called a weak electromagnetic field region. The DMB signals received in the weak electromagnetic field region have a signal level of about −30 dBm to −100 dBm. On the contrary, if the DMB receiving terminal 106 is in a metropolis with the gap filler 104 or near to it, it can receive high-level DMB signals from the gap filler 104. Accordingly the region where the DMB signals can be received from the gap filler 104 is called a strong electromagnetic field region. The DMB signals received in the strong electromagnetic field region have a signal level of about several tens to −50 dBm. Hence, it is necessary that the DMB receiving terminal 106 cover a wide range of signal levels extending from the strong electromagnetic field region to the weak electromagnetic field region.
In addition, the DMB receiving terminal 106 receives services from terrestrial DMB center 108 in the frequency band of 204 to 210 MHz. Thus, DMB services may be provided by both satellite and terrestrial broadcasting. Nevertheless, if the user of a DMB receiving terminal presently receiving satellite or terrestrial DMB services enters a weak electromagnetic field region, the DMB receiving terminal cannot receive the DMB signals, thus terminating the process of receiving the DMB services without providing advance notification of impossibility of receiving services. More specifically, because the DMB receiving terminal capable of receiving both satellite and terrestrial broadcasting can basically operate in terrestrial mode in a satellite weak electromagnetic field region, and in satellite mode in a terrestrial weak electromagnetic field region, it does not have to terminate the process of receiving the DMB services even if it enters either a satellite or terrestrial weak electromagnetic region when operating in either satellite or terrestrial mode. However, a conversion DMB receiving terminal will likely stop receiving the DMB services if it enters either satellite or terrestrial weak electromagnetic region when operating in either satellite or terrestrial mode.